The present invention relates to a memory cell of shadow RAM (Random Access Memory) employing ferroelectric capacitors, a nonvolatile memory device employing the memory cells, and a control method for the memory cell, and in particular, to shadow RAM, in which high-speed reading/writing from/to SRAM cells is realized when power is supplied thereto and nonvolatile memory by use of ferroelectric capacitors is realized when power is not supplied thereto, that is capable of operating with high reliability even when the power supply voltage is low.
A variety of shadow RAMs in which ferroelectric capacitors and SRAM cells are combined have been proposed so far. When power is being supplied, the shadow RAM stores information in its SRAM cells, offering high-speed reading/writing capability of the level of ordinary SRAM. Further, the shadow RAM realizes nonvolatile memory when power is not supplied thereto, by transferring data which has been stored in the SRAM cells to ferroelectric capacitors (as polarization directions of the ferroelectric capacitors) before the power is shut off (storing operation). In short, the shadow RAM employing ferroelectric capacitors is a storage device having two advantages: nonvolatility of ferroelectric memory and high-speed operation of SRAM.
FIG. 1 is a circuit diagram showing the composition of a memory cell of shadow RAM employing ferroelectric capacitors which has been disclosed in Japanese Patent Application Laid-Open No.2000-293989. In the memory cell of FIG. 1, a flip-flop 3 is formed by connecting two inverters 1 and 2 in ring connection (the input/output terminal of an inverter is connected to the output/input terminal of the other inverter). Two memory nodes Q0 and Q1 of the flip-flop 3 are connected to a negative bit line BLN and a positive bit line BLP respectively via NMOS transistors MO and M1 which function as transfer gates. The positive/negative bit lines BLP and BLN are used as a pair, and a sense amplifier (unshown) for comparing the voltages of the positive/negative bit lines BLP and BLN are connected to the bit lines.
To the bit lines BLP and BLN, a writing circuit (unshown) for connecting selected bit lines to the ground potential when data writing is carried out and a precharge circuit (unshown) for precharging the bit lines to a power supply voltage or the ground potential are also connected. The gates of the NMOS transistors MO and M1 are connected to a common word line WL. The word lines WL are connected to a decoder circuit (unshown). The decoder circuit selectively drives a word line as the target of access according to an address signal. Ferroelectric capacitors F0 and F1, whose lower terminals shown in FIG. 1 are connected to a common plate line PL, are connected to the memory nodes Q0 and Q1 respectively. The plate lines PL are connected to a plate line driving circuit 4. When the power is supplied to the shadow RAM, the plate line driving circuit 4 holds the voltages of the plate lines PL at Vcc/2 except in the storing operation and the recall operation of the shadow RAM.
In the following, the operation of the conventional shadow RAM employing ferroelectric capacitors will be explained in detail. Needless to say, data reading/writing from/to the flip-flops 3 of the shadow RAM are carried out in the same way as in general conventional SRAM. In idle states of the shadow RAM (in which no reading/writing is carried out), data stored in the flip-flops 3 is maintained and preserved by discharging (dropping the voltages of) all the word lines WL, precharging the bit lines to a proper voltage, and stopping the writing circuit.
When data is written into a flip-flop 3, the address decoder circuit drives (raises the voltage of a proper word line WL corresponding to the flip-flop 3, and simultaneously, the writing circuit sets one of the positive/negative bit lines BLP and BLN (corresponding to the flip-flop 3) to a low level depending on the data to be written into the flip-flop 3. By the increase of the voltage of the driven word line WL, the MOS transistors MO and M1 turn on. Since the driving power of the writing circuit is far larger than that of the inverters 1 and 2, the voltage of a memory node (Q0 or Q1) that is connected to the bit line (that is set to the low level by the writing circuit) via a MOS transistor is dropped to the ground potential. At the same time, the voltage of the other memory node (Q1 or Q0) is raised to the power supply voltage and thereby the flip-flop 3 is stabilized.
Data readout from a flip-flop 3 is carried out by precharging the bit line pair (BLP and BLN) corresponding to the flip-flop 3 to a high level, selecting and driving a proper word line corresponding to the flip-flop 3, and amplifying a voltage difference occurring between the bit line pair by use of a sense amplifier. When the voltage of the word line WL is raised, a MOS transistor (M0 or M1) that connects the low-level memory node (Q0 or Q1) and the bit line (BLN or BLP) turns on and thereby the voltage of the bit line (BLN or BLP) starts falling. The other bit line (BLP or BLN) keeps its high level since the MOS transistor (M1 or M0) does not turn on. By detecting the voltage difference between the bit line pair by use of the sense amplifier, data stored in the flip-flop 3 can be read out.
In the following, the storing operation of the shadow RAM memory cell of FIG. 1 will be explained referring to FIGS. 2 and 17. FIG. 17 shows the hysteresis characteristics of the ferroelectric capacitors F0 and F1 on a Q-V plane. FIG. 2 is a timing chart showing the change of voltage of each part of the memory cell of FIG. 1 during the storing operation. When the power is shut off, data which has been stored in the flip-flop 3 is transferred to the ferroelectric capacitors F0 and F1 and stored as polarization directions of the ferroelectric capacitors. The operation is called xe2x80x9cstoringxe2x80x9d. The storing is activated by a trigger such as a drop of the power supply voltage or a storing signal which is supplied before the power is shut off. The storing is carried out as follows.
The voltage of the plate line PL when the storing operation is started is Vcc/2. Therefore, depending on the data stored in the flip-flop 3, a voltage xe2x88x92Vcc/2 is applied to a ferroelectric capacitor that is connected to a memory node holding 0V, whereas a voltage Vcc/2 is applied to a ferroelectric capacitor that is connected to a memory node holding the power supply voltage (Vcc).
Incidentally, the aforementioned xe2x80x9cvoltagexe2x80x9d that is applied to each ferroelectric capacitor (F0, F1) is defined as a voltage difference between the upper terminal shown in FIG. 1 (which is connected to the memory node Q0 or Q1) and the lower terminal (which is connected to the plate line PL), that is, the voltage of the upper terminal relative to the lower terminal.
Subsequently, the voltage of the plate line PL is raised to Vcc. By the increase of the plate line voltage, the terminals of the latter ferroelectric capacitor (to which the voltage Vcc/2 has been applied) will have the same voltage Vcc, thereby the voltage that is applied to the ferroelectric capacitor changes to 0V. To the other ferroelectric capacitor, a voltage xe2x88x92Vcc is applied, and thereby the status of the ferroelectric capacitor gets to a point C in the hysteresis loop of FIG. 17.
Subsequently, the voltage of the plate line PL is dropped to 0V, thereby a voltage Vcc is applied to the ferroelectric capacitor that is connected to the memory node holding Vcc and thereby the status of the ferroelectric capacitor gets to a point A in the hysteresis loop of FIG. 17. At the same time, the ferroelectric capacitor that has been at the point C moves to a point D and holds negative remanent polarization.
Finally, the power is shut off. After the power shutoff, the voltage of each memory node converges on the ground potential. Consequently, the ferroelectric capacitor that has been at the point A moves to a point B and holds positive remanent polarization. The remanent polarization of the ferroelectric capacitor endures more than ten years when no voltage is applied thereto, thereby nonvolatile memory is realized in the conventional shadow RAM employing ferroelectric capacitors.
In the following, the recall operation of the shadow RAM memory cell of FIG. 1 will be explained referring to FIG. 3. FIG. 3 is a timing chart showing the change of voltage of each part of the memory cell of FIG. 1 during the recall operation. When the power is turned on, the data which has been stored in the ferroelectric capacitors is transferred to the flip-flop 3. The operation is called xe2x80x9crecallxe2x80x9d. When the power is turned on, the data which has been memorized as the remanent polarization of the ferroelectric capacitors can be recalled to the flip-flop 3, only by supplying power to the flip-flop 3 while maintaining the word line WL and the plate line PL at the low level. As the supply voltage to the flip-flop 3 increases, the voltages of the memory nodes also increase due to the coupling of MOS transistors of the inverters 1 and 2, thereby the voltages applied to the ferroelectric capacitors increase from 0V.
The ferroelectric capacitor holding the positive remanent polarization at the point B of FIG. 17 functions as a smaller capacitance than the ferroelectric capacitor holding the negative remanent polarization at the point D. That is evident from the gentler slope of the path from the point B to the point A on the Q-V plane (shown by an arrow Y1) in comparison with the slope of the path from the point D to the point A (shown by an arrow Y2). Therefore, one of the memory nodes to which the former ferroelectric capacitor (smaller capacitance) is connected raises its voltage faster than the other memory node.
The supply voltage of the flip-flop 3 keeps on increasing, and when the voltage of one of the memory nodes exceeds the threshold voltage of transistors of the inverters 1 and 2, positive feedback is applied to the flip-flop 3 and thereby the voltage difference between the memory nodes is enhanced or amplified. Consequently, the voltage of the memory node corresponding to (that is connected to) the ferroelectric capacitor that stayed at the point B becomes Vcc, whereas the voltage of the other memory node corresponding to the ferroelectric capacitor that stayed at the point D becomes GND. Finally, the voltage of the plate line PL is set to Vcc/2 and the idle state is started. Consequently, the ferroelectric capacitor that had been holding the voltage Vcc before the power shutoff thereafter holds its data at the point B, and holds the voltage Vcc again after the power is turned on again. Similarly, the ferroelectric capacitor that had been holding 0V before the power shutoff thereafter holds its data at the point D, and holds 0V also after the power is turned on again.
As explained above, in the conventional shadow RAM employing ferroelectric capacitors, the data stored in the flip-flop 3 is maintained and preserved after the power shutoff and power on and thereby nonvolatile memory is realized. In addition, data reading/writing can be carried out in the same way as ordinary SRAM since the flip-flop 3 and the MOS transistors M0 and M1 operate similarly to an ordinary SRAM cell.
In some known nonvolatile memories employing ferroelectric capacitors, memory cells like those disclosed in Japanese Patent No.2674775 are employed and each memory cell is composed of a combination of a transistor and a ferroelectric capacitor or a combination of two transistors and two ferroelectric capacitors. In nonvolatile memory of such type, data is memorized as the polarization direction of the ferroelectric capacitor (or as the polarization directions of the ferroelectric capacitors) regardless of whether the power is being supplied or not. Further, data readout in such nonvolatile memory is destructive readout, and thus data has to be written again after the readout. Access to each ferroelectric capacitor becomes very frequent due to the repetitive data writing, therefore, enough device reliability after long-term use can hardly be guaranteed by the present manufacturing technology. On the other hand, in the shadow RAM employing ferroelectric capacitors, access to the ferroelectric capacitor occurs only in the storing operation and the recall operation, therefore, enough product reliability can be ensured even if the quality of the ferroelectric capacitors is relatively low.
FIG. 4 is a circuit diagram showing the composition of a conventional semiconductor memory of Japanese Patent Application Laid-Open No. HEI9-17965, in which RAM having the nonvolatile memory function is disclosed. The memory cell MC of FIG. 4 is composed of transfer MISFETs Qt1 and QT2, an SRAM memory cell that is composed of a flip-flop circuit, and ferroelectric capacitors Cf1 and Cf2. As shown in FIG. 4, the flip-flop circuit of the memory cell MC is composed of two N-channel MISFETs (driving MISFETs) Qd1 and Qd2 and two P-channel MISFETs (load MISFETs) Qp1 and Qp2.
The transfer MISFET Qt1 connects a memory node N, of the flip-flop circuit to a data line DL1, and the other transfer MISFET Qt2 connects another memory node N2 of the flip-flop circuit to another data line DL2. The gates of the transfer MISFETs Qt1 and QT2 are connected to a word line WL.
Each memory node (N1, N2) is connected to an electrode of a corresponding ferroelectric capacitor (Cf1, Cf2), and the other electrodes of the ferroelectric capacitors Cf1 and Cf2 are electrically connected together at a node N3. To the node N3, a plate voltage (VP) is applied.
In the following, data readout from the flip-flop circuit to the ferroelectric capacitors Cf1 and Cf2 will be explained referring to FIGS. 5 and 6. Incidentally, FIG. 7 shows the change of the supply voltage VL of the flip flop circuit and the plate voltage VP and FIG. 8 shows the change of the voltages of the memory nodes N1 and N2.
If data stored in the flip-flop circuit has to be transferred to the ferroelectric capacitors Cf1 and Cf2 at a time t1, the supply voltage VL of the flip-flop circuit is increased from Vcc to Vccxe2x80x2 as shown in FIG. 5 and FIG. 7 while maintaining the plate voltage VP at Vss (low level). Incidentally, in this explanation, the data which has been stored in the flip-flop circuit at the time t1 is assumed to be: (memory node N1, memory node N2)=(high level, low level)=(Vccxe2x80x2, Vss).
The voltage Vccxe2x80x2 is assumed to be a high voltage enough to reverse the polarization of the ferroelectric capacitors Cf1 and Cf2. Since the node N3 is at the low level (Vss), by the voltage increase of the memory node N1, a positive-polarization state is written into the ferroelectric capacitor Cf1 which is connected to the memory node N1 as shown in FIG. 5.
Next, data transfer has to be done to the ferroelectric capacitor Cf2 which is connected to the memory node N2. At a time T2, the plate voltage is increased from Vss to Vccxe2x80x2 and thereby the node N3 is raised to the high level (Vccxe2x80x2) while maintaining the supply voltage of the flip-flop circuit at Vccxe2x80x2. Since the memory node N2 is at the low level (Vss), a negative-polarization state is written into the ferroelectric capacitor Cf2 which is connected to the memory node N2 as shown in FIG. 6.
Even if all the voltages became 0V at a time t3 and thereby the data of the memory nodes N1 and N2 disappeared, the polarization of the ferroelectric capacitors Cf1 and Cf2 remains, therefore, the data of the flip-flop circuit can be maintained and preserved in the ferroelectric capacitors Cf1 and Cf2.
Next, data writing from the ferroelectric capacitors Cf1 and Cf2 to the flip-flop circuit will be explained referring to FIGS. 9 through 13.
If data stored in the ferroelectric capacitors Cf1 and Cf2 has to be transferred to the flip-flop circuit at a time t4, the plate voltage VP is increased from Vcc to Vccxe2x80x2 while maintaining the supply voltage of the flip-flop circuit at Vss. Since the supply voltage is set to Vss, the load MISFETs Qp1 and Qp2 remain in off states.
However, at the time t4, current passes from the load MISFET Qp1 and the driving MISFET Qd1 to the memory node N1 and thereby the voltage of the memory node N1 rises to VN1 instantaneously. The voltage level VN1 is determined by the capacitances of the ferroelectric capacitors Cf1 and Cf2 and the parasitic capacitances of the load MISFETs Qp1 and Qp2 and the driving MISFETs Qd1 and Qd2.
When the voltages of the memory nodes N1 and N2 rise into VN1 and the voltage level VN1 exceeds the threshold voltage of the driving MISFETs Qd1 and Qd2, the driving MISFETs Qd1 and Qd2 turn on, thereby current passes from the memory node N1 to the driving MISFET Qd1 and thereby the voltage of the memory node N1 drops to almost 0V. Similarly, current passes from the memory node N2 to the driving MISFET Qd2 and thereby the voltage of the memory node N2 drops to almost 0V
Consequently, the state of the ferroelectric capacitor Cf1, which has been in the positive-polarization state at the time t4, is changed to the negative-polarization state. Incidentally, the ferroelectric capacitor Cf2, which has been in the negative-polarization state at the time t4, remains in the negative-polarization state.
When the polarization of the ferroelectric capacitor Cf1 reverses, a polarization reversal current passes and thereby the voltage of the memory node N1 (VN2) gets higher than that of the memory node N2 (VN3), that is, a voltage difference occurs between the memory nodes N1 and N2. In such a state, if the supply voltage of the flip-flop circuit is raised to Vccxe2x80x2 at a time t6, a positive feedback is applied to the flip-flop circuit, thereby the memory nodes N1 and N2 are set to the high level (Vccxe2x80x2) and the low level (Vss), respectively.
Subsequently, the plate voltage is dropped to Vss at a time t7 and thereby the state of the ferroelectric capacitor Cf1, which has been in the negative-polarization state at the time t6, is changed to the positive-polarization state. Thereafter, the supply voltage of the flip-flop circuit is dropped to Vcc at a time t8, thereby the voltage of the memory node N1 is changed from Vccxe2x80x2 to Vcc and thereby the flip-flop circuit returns to its normal operation state.
The normal operation of the flip-flop circuit, the data readout from the flip-flop circuit to the ferroelectric capacitors Cf1 and Cf2, and the data writing from the ferroelectric capacitors Cf1 and Cf2 to the flip-flop circuit are carried out as explained above.
However, the conventional shadow RAM and semiconductor memory employing ferroelectric capacitors which have been explained above involves the following problems or drawbacks.
In the conventional shadow RAM employing ferroelectric capacitors, the plate line voltage is changed between the ground potential and the power supply voltage (Vcc) in the storing operation, thereby voltages (Vcc or xe2x88x92Vcc) according to data to be stored are applied to the ferroelectric capacitors and thereby positive/negative remanent polarization is caused. The power supply voltage (Vcc) has to be applied to the ferroelectric capacitor for realizing the nonvolatile memory in the conventional shadow RAM, therefore, if the power supply voltage (Vcc) decreased due to the miniaturization of the integrated circuit, the application of enough voltage to the ferroelectric capacitor becomes difficult and thereby the reliability of the nonvolatile memory data is necessitated to be deteriorated.
Meanwhile, in the conventional semiconductor memory of Japanese Patent Application Laid-Open No. HEI9-17965, the power supply voltage of the memory cell is increased from the ordinary power supply voltage Vcc to a higher power supply voltage Vccxe2x80x2 in order to write the data stored in the flip-flop circuit into the ferroelectric capacitors and in order to write the data stored in the ferroelectric capacitors into the flip-flop circuit as explained above. Due to the need of increasing the power supply voltage of the memory cell into a voltage higher than the ordinary power supply voltage Vcc, the use of generally-used high-performance devices becomes difficult or impossible.
It is therefore the primary object of the present invention to provide a memory cell of shadow RAM employing ferroelectric capacitors, a nonvolatile memory device, and a control method for the memory cell, by which the storing operation can be carried out with high reliability even if the power supply voltage decreased.
Another object of the present invention is to provide a memory cell of shadow RAM employing ferroelectric capacitor, a nonvolatile memory device, and a control method for the memory cell, by which the recall operation can be carried out with high reliability even if the power supply voltage decreased.
In accordance with a first aspect of the present invention, there is provided a memory cell comprising: a pair of memory nodes for holding a pair of complementary voltages; a pair of switching elements for controlling the connection between each memory node and a bit line corresponding to the memory node according to ON/OFF control by a common word line; and a pair of ferroelectric capacitors each of which is connected to a plate line and corresponding one of the memory nodes. Storing operation of the memory cell is carried out by swinging the voltage of the plate line between a first voltage that is higher than power supply voltage of the memory cell and a second voltage that is lower than the ground potential while keeping the pair of switching elements in off states (hereafter, referred to as xe2x80x9cfirst storing operationxe2x80x9d).
By driving the plate line to the first voltage that is higher than the power supply voltage in the storing operation, a negative bias voltage (ground potentialxe2x80x94first voltage) that is lower (higher in the absolute value) than the inverse of the power supply voltage (xe2x80x94power supply voltage) can be applied to a ferroelectric capacitor that is connected to a memory node holding data at the ground potential. By the enhancement of the negative bias voltage, nonvolatile memory with high reliability can be realized even if the power supply voltage became lower due to the miniaturization of the integrated circuit etc.
By driving the plate line to the second voltage lower than the ground potential in the storing operation, a positive bias voltage (power supply voltagexe2x80x94second voltage) that is higher than the power supply voltage can be applied to a ferroelectric capacitor that is connected to a memory node holding data at the power supply voltage. By the enhancement of the positive bias voltage, nonvolatile memory with high reliability can be realized even if the power supply voltage became lower due to the miniaturization of the integrated circuit etc.
In accordance with a second aspect of the present invention, storing operation of the memory cell is carried out by swinging the voltage of the plate line between power supply voltage of the memory cell and a second voltage that is lower than the ground potential while keeping the pair of switching elements in off states (hereafter, referred to as xe2x80x9csecond storing operationxe2x80x9d).
In accordance with a third aspect of the present invention, storing operation of the memory cell is carried out by swinging the voltage of the plate line between a first voltage that is higher than power supply voltage of the memory cell and the ground potential while keeping the pair of switching elements in off states (hereafter, referred to as xe2x80x9cthird storing operationxe2x80x9d).
In accordance with a fourth aspect of the present invention, recall operation of the memory cell is carried out by driving the plate line to a third voltage that is higher than power supply voltage of the memory cell and thereafter raising supply voltage of the memory cell from the ground potential to the power supply voltage of the memory cell while keeping the pair of switching elements in off states (hereafter, referred to as xe2x80x9cfirst recall operationxe2x80x9d).
By driving the plate line to the third voltage (higher than the power supply voltage) keeping the supply voltage of the memory cell (flip-flop) at the ground potential (keeping switching elements in off states) in the recall operation, negative voltages are applied to the ferroelectric capacitors and thereby a voltage difference is caused to the pair of memory nodes. Thereafter, by raising the supply voltage of the memory cell (flip-flop), the voltage difference between the memory nodes is amplified and thereby the data is recalled and restored. By raising the plate line voltage to the high voltage (third voltage), higher bias voltages can be applied to the ferroelectric capacitors in comparison with conventional shadow RAM raising the plate line voltage to the power supply voltage (Japanese Patent No.2693967, for example), thereby nonvolatile memory with high reliability can be realized even if the power supply voltage became lower due to the miniaturization of the integrated circuit etc.
In accordance with a fifth aspect of the present invention, recall operation of the memory cell is carried out by driving the plate line to a fourth voltage that is lower than the ground potential and raising supply voltage of the memory cell from the ground potential to the power supply voltage of the memory cell while keeping the pair of switching elements in off states (hereafter, referred to as xe2x80x9csecond recall operationxe2x80x9d).
By driving the plate line to the fourth voltage (lower than the ground potential) concurrently with the raising of the supply voltage of the memory cell (flip-flop), positive voltages are applied to the ferroelectric capacitors and thereby a voltage difference is caused to the pair of memory nodes. Thereafter, by raising the supply voltage of the memory cell (flip-flop), the voltage difference between the memory nodes is amplified and thereby the data is recalled and restored. By dropping the plate line voltage to the negative voltage (fourth voltage), higher bias voltages can be applied to the ferroelectric capacitors in comparison with conventional shadow RAM fixing the plate line voltage to the ground potential (Japanese Patent Application Laid-Open No.2000-293989, for example), thereby nonvolatile memory with high reliability can be realized even if the power supply voltage became lower due to the miniaturization of the integrated circuit etc.
In accordance with a sixth aspect of the present invention, in the first aspect, recall operation of the memory cell is carried out according to the first recall operation.
In accordance with a seventh aspect of the present invention, in the sixth aspect, the third voltage is set equal to the first voltage.
In accordance with an eighth aspect of the present invention, in the first aspect, recall operation of the memory cell is carried out according to the second recall operation.
In accordance with a ninth aspect of the present invention, in the eighth aspect, the fourth voltage is set equal to the second voltage.
In accordance with a tenth aspect of the present invention, in the second aspect, recall operation of the memory cell is carried out according to the first recall operation.
In accordance with an eleventh aspect of the present invention, in the second aspect, recall operation of the memory cell is carried out according to the second recall operation.
In accordance with a twelfth aspect of the present invention, in the eleventh aspect, the fourth voltage is set equal to the second voltage.
In accordance with a thirteenth aspect of the present invention, in the third aspect, recall operation of the memory cell is carried out according to the first recall operation.
In accordance with a fourteenth aspect of the present invention, in the thirteenth aspect, the third voltage is set equal to the first voltage.
In accordance with a fifteenth aspect of the present invention, in the third aspect, recall operation of the memory cell is carried out according to the second recall operation.
In accordance with a sixteenth aspect of the present invention, in the sixth aspect, the memory cell includes a pair of logic inversion elements which are connected together in ring connection so that each of the memory nodes will be formed between the output terminal of one logic inversion element and the input terminal of the other logic inversion element.
In accordance with a seventeenth aspect of the present invention, in the sixteenth aspect, the switching elements are implemented by MOS transistors.
In accordance with an eighteenth aspect of the present invention, in the sixteenth aspect, the switching elements and the logic inversion elements are implemented by a 6-transistor CMOS flip-flop.
In accordance with a nineteenth aspect of the present invention, in the sixteenth aspect, the switching elements and the logic inversion elements are implemented by a high-resistance-load 4-transistor flip-flop.
In accordance with a twentieth aspect of the present invention, in the sixteenth aspect, the switching elements and the logic inversion elements are implemented by a loadless 4-transistor flip-flop.
In accordance with a twenty-first aspect of the present invention, in the eighth aspect, the memory cell includes a pair of logic inversion elements which are connected together in ring connection so that each of the memory nodes will be formed between the output terminal of one logic inversion element and the input terminal of the other logic inversion element.
In accordance with a twenty-second aspect of the present invention, in the twenty-first aspect, the switching elements are implemented by MOS transistors.
In accordance with a twenty-third aspect of the present invention, in the twenty-first aspect, the switching elements and the logic inversion elements are implemented by a 6-transistor CMOS flip-flop.
In accordance with a twenty-fourth aspect of the present invention, in the twenty-first aspect, the switching elements and the logic inversion elements are implemented by a high-resistance-load 4-transistor flip-flop.
In accordance with a twenty-fifth aspect of the present invention, in the twenty-first aspect, the switching elements and the logic inversion elements are implemented by a loadless 4-transistor flip-flop.
In accordance with a twenty-sixth aspect of the present invention, there is provided a nonvolatile memory device comprising memory cells that are arranged in a matrix. The memory cell includes: a pair of memory nodes for holding a pair of complementary voltages; a pair of switching elements for controlling the connection between each memory node and a bit line corresponding to the memory node according to ON/OFF control by a common word line; and a pair of ferroelectric capacitors each of which is connected to a plate line and corresponding one of the memory nodes. In the nonvolatile memory device, at least storing operation or recall operation of each memory cell is carried out according to operation selected from: (A) first storing operation in which the voltage of the plate line is swung between a first voltage that is higher than power supply voltage of the memory cell and a second voltage that is lower than the ground potential while keeping the pair of switching elements in off states; (B) second storing operation in which the voltage of the plate line is swung between the power supply voltage of the memory cell and a second voltage that is lower than the ground potential while keeping the pair of switching elements in off states; (C) third storing operation in which the voltage of the plate line is swung between a first voltage that is higher than the power supply voltage of the memory cell and the ground potential while keeping the pair of switching elements in off states; (D) first recall operation in which the plate line is driven to a third voltage that is higher than power supply voltage of the memory cell and thereafter supply voltage of the memory cell is raised from the ground potential to the power supply voltage of the memory cell while keeping the pair of switching elements in off states; and (E) second recall operation in which the plate line is driven to a fourth voltage that is lower than the ground potential and supply voltage of the memory cell is raised from the ground potential to the power supply voltage of the memory cell while keeping the pair of switching elements in off states.
In accordance with a twenty-seventh aspect of the present invention, in the twenty-sixth aspect, all the memory cells are connected to a common plate line.
In accordance with a twenty-eighth aspect of the present invention, in the twenty-seventh aspect, the nonvolatile memory device further comprises a plate line driving circuit for driving the common plate line.
In accordance with a twenty-ninth aspect of the present invention, in the twenty-eighth aspect, the nonvolatile memory device further comprises a high voltage generation circuit for generating a voltage higher than the power supply voltage and supplying the high voltage to the plate line driving circuit.
In accordance with a thirtieth aspect of the present invention, in the twenty-eighth aspect, the nonvolatile memory device further comprises a negative voltage generation circuit for generating a negative voltage and supplying the negative voltage to the plate line driving circuit.
In accordance with a thirty-first aspect of the present invention, in the twenty-ninth aspect, the nonvolatile memory device further comprises a negative voltage generation circuit for generating a negative voltage and supplying the negative voltage to the plate line driving circuit.
In accordance with a thirty-second aspect of the present invention, in the twenty-ninth aspect, the memory cells are implemented by devices of normal withstand voltages, and the plate line driving circuit and the high voltage generation circuit are implemented by devices of high withstand voltages.
In accordance with a thirty-third aspect of the present invention, in the thirtieth aspect, the memory cells are implemented by devices of normal withstand voltages, and the plate line driving circuit and the negative voltage generation circuit are implemented by devices capable of operating under negative voltages.
In accordance with a thirty-fourth aspect of the present invention, in the twenty-eighth aspect, the plate line driving circuit raises the voltage of the common plate line from a preset voltage that is between the ground potential and the power supply voltage to the first voltage and thereafter drops the voltage to the second voltage in the storing operation.
In accordance with a thirty-fifth aspect of the present invention, in the twenty-eighth aspect, the plate line driving circuit drops the voltage of the common plate line from a preset voltage that is between the ground potential and the power supply voltage to the second voltage and thereafter raises the voltage to the first voltage in the storing operation.
In accordance with a thirty-sixth aspect of the present invention, in the twenty-eighth aspect, the plate line driving circuit raises the voltage of the common plate line from a preset voltage that is between the ground potential and the power supply voltage to the power supply voltage and thereafter drops the voltage to the second voltage in the storing operation.
In accordance with a thirty-seventh aspect of the present invention, in the twenty-eighth aspect, the plate line driving circuit drops the voltage of the common plate line from a preset voltage that is between the ground potential and the power supply voltage to the second voltage and thereafter raises the voltage to the power supply voltage in the storing operation.
In accordance with a thirty-eighth aspect of the present invention, in the twenty-eighth aspect, the plate line driving circuit raises the voltage of the common plate line from a preset voltage that is between the ground potential and the power supply voltage to the first voltage and thereafter drops the voltage to the ground potential in the storing operation.
In accordance with a thirty-ninth aspect of the present invention, in the twenty-eighth aspect, the plate line driving circuit drops the voltage of the common plate line from a preset voltage that is between the ground potential and the power supply voltage to the ground potential and thereafter raises the voltage to the first voltage in the storing operation.
In accordance with a fortieth aspect of the present invention, in the twenty-eighth aspect, the plate line driving circuit raises the voltage of the common plate line to the third voltage before the supply voltage of the memory cells is raised in the recall operation.
In accordance with a forty-first aspect of the present invention, in the twenty-eighth aspect, the plate line driving circuit drops the voltage of the common plate line to the fourth voltage almost concurrently with the raising of the supply voltage of the memory cells in the recall operation.
In accordance with a forty-second aspect of the present invention, there is provided a control method for a memory cell that comprises: a pair of memory nodes for holding a pair of complementary voltages; a pair of switching elements for controlling the connection between each memory node and a bit line corresponding to the memory node according to ON/OFF control by a common word line; and a pair of ferroelectric capacitors each of which is connected to a plate line and corresponding one of the memory nodes. In the control method, storing operation of the memory cell is carried out by swinging the voltage of the plate line between a first voltage that is higher than power supply voltage of the memory cell and a second voltage that is lower than the ground potential while keeping the pair of switching elements in off states (first storing operation).
In accordance with a forty-third aspect of the present invention, storing operation of the memory cell is carried out by swinging the voltage of the plate line between power supply voltage of the memory cell and a second voltage that is lower than the ground potential while keeping the pair of switching elements in off states (second storing operation).
In accordance with a forty-fourth aspect of the present invention, storing operation of the memory cell is carried out by swinging the voltage of the plate line between a first voltage that is higher than power supply voltage of the memory cell and the ground potential while keeping the pair of switching elements in off states (third storing operation).
In accordance with a forty-fifth aspect of the present invention, recall operation of the memory cell is carried out by driving the plate line to a third voltage that is higher than power supply voltage of the memory cell and thereafter raising supply voltage of the memory cell from the ground potential to the power supply voltage of the memory cell while keeping the pair of switching elements in off states (first recall operation).
In accordance with a forty-sixth aspect of the present invention, recall operation of the memory cell is carried out by driving the plate line to a fourth voltage that is lower than the ground potential and raising supply voltage of the memory cell from the ground potential to the power supply voltage of the memory cell while keeping the pair of switching elements in off states (second recall operation).
In accordance with a forty-seventh aspect of the present invention, in the forty-second aspect, recall operation of the memory cell is carried out according to the first recall operation.
In accordance with a forty-eighth aspect of the present invention, in the forty-seventh aspect, the third voltage is set equal to the first voltage.
In accordance with a forty-ninth aspect of the present invention, in the forty-second aspect, recall operation of the memory cell is carried out according to the second recall operation.
In accordance with a fiftieth aspect of the present invention, in the forty-ninth aspect, the fourth voltage is set equal to the second voltage.
In accordance with a fifty-first aspect of the present invention, in the forty-third aspect, recall operation of the memory cell is carried out according to the first recall operation.
In accordance with a fifty-second aspect of the present invention, in the forty-third aspect, recall operation of the memory cell is carried out according to the second recall operation.
In accordance with a fifty-third aspect of the present invention, in the fifty-second aspect, the fourth voltage is set equal to the second voltage.
In accordance with a fifty-fourth aspect of the present invention, in the forty-fourth aspect, recall operation of the memory cell is carried out according to the first recall operation.
In accordance with a fifty-fifth aspect of the present invention, in the fifty-fourth aspect, the third voltage is set equal to the first voltage.
In accordance with a fifty-sixth aspect of the present invention, in the forty-fourth aspect, recall operation of the memory cell is carried out according to the second recall operation.